Where did Achan hide the things he took from Jericho?

Answer: In the ground in his tent

Achan confessed that the cloak, silver and gold bar were hidden in the ground inside his tent, with the silver underneath.

“When I saw among the spoils a beautiful cloak from Shinar, two hundred shekels of silver, and a bar of gold weighing fifty shekels, I coveted them and took them. They are hidden in the ground inside my tent, with the silver underneath.””— Joshua 7:21 (BSB)